With the advent of iOS 15, Safari (for iOS) now supports full blown extensions, just like Safari on the desktop. As such, AgileBits updated 1Password (to 7.8) shortly after the iOS 15 release that included a new Safari extension. Unfortunately, as the name implies, these Safari extension only work in Safari and not 3rd party browsers or apps (even though all browsers and apps must use the Safari webkit engine). Firefox is stuck using Apple's built-in autofill feature, which is why you are seeing such a drastic difference.
On a more frustrating note, prior to 7.8, 1P supported not only password autofill, but also a share sheet extension that allowed you to fill in non-password fields (such as credit card #'s and addresses), This share sheet extension worked not only in Safari, but in 3rd party apps and browsers. However, when 1P 7.8 was released with the Safari extension, AgileBits decided to remove the share sheet extension. As a result, 1P functionality has become unnecessarily limited in 3rd party apps.
The long and the short of all of this is two-fold: (1) 1Password's Safari extension can only work in Safari. This is not a limitation of 1Password, but a limitation of iOS itself. Until Apple allows extensions in 3rd party browsers, you're stuck with the more generic autofill features. (2) 1Password previously supported autofilling non-password fields by utilizing iOS' built-in share sheet extension framework. However, that extension has now been removed and thus you are limited to password-only fields in 3rd party apps and extensions. This is 100% a limitation of 1Password and has nothing to do with any iOS limitation. If AgileBits wanted to bring back the share sheet extension they easily could in a future update (this won't happen, however, as the new Safari extension was designed, in part, to drive standalone license users into the subscription model... but that's an entirely different discussion that has been ongoing since AgileBits announced 1Password 8).
In any case, hopefully that helps explain everything!